As you may know, a server may return a 403 Forbidden status in response to a request from a client or it may indicate that the server can be reached and process the request but refuses to take any further action.

Please navigate to ~/.activecho on the Mac machine and check the logs, they may contain some clues. Also, since it looks like a return error from the server, checking the Access server logs (they are located at C:\Program Files (x86)\Acronis\Access\Common\apache-tomcat-7.0.42\logs by default) can be helpful too.

Other than that, you can open a support ticket at https://support.acronis.com/mobility.